The influence of natural zeolites on the workability and setting time of cement-based construction materials was also investigated. The workability while in the plastic state can affect the hardened properties of cement-based materials. A stiff mix may require excessive vibration to fully and evenly fill the formwork, which may lead to segregation. On the other hand, a fluid mix may result in segregation and water bleeding. Both these extreme scenarios should be avoided and careful investigations should be conducted in designing a cohesive mix but with sufficient workability.

A recent study investigated the influence of clinoptilolite zeolite on the slump and setting time values of mortar and cement paste, respectively. Increasing the zeolite content to 10% resulted in a 25% reduction in workability compared to the control mix [3]. At the same time, the initial and final setting times of the cement pastes showed a decreasing trend with the increase in the replacement percentage. One possible explanation could be the higher specific area of zeolite particles coupled with the porous nature of zeolites, which results in part of the mixing water being absorbed. The effect of zeolites on the fresh properties of the investigated cement-based materials was more pronounced compared to other supplementary cementitious materials used in the study: metakaolin, ground granulated blast furnace slag and type C fly-ash.

In another study conducted on the pore structure of cement pastes containing natural zeolites, it was reported that the initial setting time values decreased with the increase in zeolite content, while the final setting time values increased. The considered cement replacement percentages by clinoptilolite zeolite were 10%, 20% and 30%, by mass. A rather high water/binder ratio of 0.6 was chosen, which could explain the longer final setting time values. Moreover, the initial setting time values did not change with the increase in the replacement percentage, while higher percentages than 20% had no influence on the final setting time. The flow, however, decreased with the increase in the zeolite content in an almost linear manner [4].

Different results were reported in [10] in terms of initial and final setting time values of cement pastes containing zeolites. The authors concluded that the variation between the setting time values of all considered mixes was small and, therefore, the zeolite content did not play a significant role. However, a 13.79% increase in the initial setting time value was reported for a replacement percentage of 20%. An interesting conclusion was drawn in terms of the volume stability of zeolite mixes, especially for replacement percentages equal to and higher than 10%. These mixes showed a better volume stability compared to the reference mix, which could be attributed to the volume stability of zeolite particles [5] and to the significantly reduced shrinkage provided by the presence of zeolites [11].

Calcination of zeolites to 600 °C and 800 °C results in a lower demand for superplasticizer, at a constant water/binder ratio of 0.33. The calcination leads to lower water demand because the porosity of zeolites is greatly reduced [13]. At the same time, while the amorphous content of zeolites increases with calcination temperature, their pozzolanic activity, porosity and surface area greatly reduced [13,14].

The use of natural zeolites together with limestone powder as substitutes for cement in self-compacting mortar resulted in a higher dosage of water-reducing admixture coupled with longer times for the mortar to reach spread diameters of 250 mm and 300 mm [15]. The study concluded that the combined use of zeolites with limestone powder in equal parts, at a constant water/cement ratio of 0.42 and a similar water/binder ratio of 0.336, resulted in overall better characteristics of self-compacting mortar than all other blends of supplementary cementitious materials investigated in the study.

In a different study, the cement was replaced by zeolite at rates of 5%, 10%, 15% and 20%, by volume. The results showed an increase in the flowability of mortar with up to 10% replacement, compared to the reference mix. Higher replacement percentages resulted in lower flowability values but all zeolite-containing mixes exhibited larger values compared to the control mix [9]. The porous nature of zeolite coupled with its high specific area were considered as the main influencing factors for the obtained results for replacement percentages higher than 10%. A previous study also indicated that another possible cause could be the angular shape of zeolite particles, which may result in increased friction forced between the particles of the mix [16].

In an earlier study, it was found that replacing Portland cement by amounts of 5%, 10%, 15% and 20%, by mass, resulted in a higher dosage of superplasticizer in order to obtain a similar slump with the reference concrete mix. At the same time, the air content of the mix increased with the increase in the replacement percentage due to the porous nature of zeolites [2].

Similar results were reported in [17] for 10% and 15% replacement percentages by mass of cement. Increasing the water/binder (w/b) ratio from 0.35 to 0.5 resulted in lower dosages of water-reducing admixture. However, the increased content of water-reducing admixture after increasing the replacement percentage was reported for all considered w/b ratios.

This section summarizes recent findings in terms of the compressive strength, flexural strength and modulus of elasticity of cement-based construction materials. The information is presented and discussed in terms of influencing factors such as the role of natural zeolites as supplementary cementitious materials or aggregates, the replacement percentages of Portland cement or traditional aggregates and the water/cementitious binder ratio, as well as the curing age of tested specimens. Where applicable, the use of natural zeolites together with other pozzolans will be presented and discussed.

There are not many studies addressing the mechanical properties of cement pastes with zeolites. Still, the findings are similar to the ones reported for mortars and concrete.

In a comprehensive research work, six different water/binder ratios, four replacement percentages of cement by natural zeolite and three different curing ages were considered for cement pastes in order to assess the compressive strength [18]. The highest gain in compressive strength values were obtained from 7 days to 28 days, irrespective of zeolite content or water/binder ratio. However, the strength gain from 28 days to 70 days was more pronounced for zeolite-containing pastes than for the reference ones. This gain was mostly governed by the zeolite content, whereas the w/b ratio had little effect. This confirmed the slow pozzolanic reaction of clinoptilolite zeolite reported in other studies [3,7].

There was no clear increasing or decreasing trend in terms of compressive strength values up to the age of 28 days for pastes containing zeolites compared to the reference mix. However, at the age of 70 days, all pastes with zeolites showed consistently larger values compared to the reference mixes.

In the case of mortars, the mechanical properties are determined on 40 mm × 40 mm × 160 mm prisms. The specimens are subjected to three-point loading tests to determine the flexural tensile strength. The compressive strength is determined from the uniaxial compression tests conducted either on the resulting half prisms from the bending test or on cube specimens.

The slow pozzolanic reaction of clinoptilolite zeolites results in lower flexural strength and compressive strength values of mortars at the age of 28 days. When samples are subjected to elevated temperatures (200 °C, 300 °C, 400 °C, 650 °C and 800 °C), the presence of zeolites increases the possibility of higher hydration production occurrence due to water release from the porous structure of the zeolite coupled with internal pore pressure, which leads to the so-called autoclave curing [20]. This effect was observed especially at temperatures higher than 400 °C.

Researchers looked for alternatives to improve the effect of zeolites as supplementary cementitious materials. There are currently two main approaches: milling, to decrease the particle size, and calcination, which helps in reducing their porous structure and, consequently, decreasing the water demand [13,21]. According to a recent study, calcination of natural zeolites resulted in marginal improvements over non-calcined zeolites in terms of compressive strength at either early age or 28 days. Milling pre-treatment, on the other hand, resulted in significantly improved values of the compressive strength of zeolites containing mortars, compared to the natural, non-treated zeolite mortar. The combination between milling and calcination pre-treatments resulted only in marginal gains that could not justify the embed energy consumption [21].

According to a recent study, the use of zeolites blended with other supplementary cementitious materials, at water/binder ratios lower than 0.45, resulted in improved values of mechanical properties [15]. The TGA analysis showed that blending limestone powder with natural zeolites resulted in better hydration compared to the control mix, although the cement content was lower.

The use of nano-silica in cement-based mortar promotes the acceleration of cement hydration. Using blends of nano-silica and zeolites to replace the cement not only results in higher compressive strength values at early ages but also beyond 28 days when the pozzolanic reaction of zeolites starts contributing [22]. The small dimensions of nano-silica create a nucleation site inside the matrix and lead to the formation of a denser structure. Hence, it has been hypothesized that calcium hydroxide crystals have less space to grow which results in smaller dimensions but larger numbers [23,24]. This leads to an increase in the lateral surface of CH crystals, accelerating the pozzolanic reaction. Therefore, it can be assumed that the use of nano-silica together with zeolites, or any other natural pozzolan, has a synergistic effect because their beneficial effects are augmented by the presence of the nano-silica.

The use of zeolites as a replacement for aggregates, e.g., sand, in engineering cementitious composites was considered from the point of view of the zeolite’s internal curing properties [25] rather than the strength gains of the resulting material. A successful decrease in the 28 day shrinkage was obtained at the cost of a 10% reduction in compressive strength for a 30% replacement, by mass, of quartz sand by natural zeolite.

In the case of concrete, studies revealed that a smaller zeolite particle size resulted in higher compressive strength values, irrespective of the considered curing age of concrete [27]. A lower w/b ratio resulted in a better performance of zeolites containing self-compacting concrete, which had higher compressive strength values compared to the reference mix [28]. The obtained results are in line with previously reported trends [29].

The increase in the compressive strength of zeolites containing concrete could be attributed to the active SiO₂ and Al₂O₃ present in the zeolite, promoting its pozzolanic activity [30].

After 90 days of curing, the hydration of 20% zeolite concrete resulted in a compact microstructure of the matrix, as demonstrated by means of mercury intrusion porosimetry (MIP) results. The total porosity was lower than that of the control mix. Increasing the replacement percentage resulted in large capillary pores and cracks occurring in the concretes [31].

Due to the widespread application of concrete in civil engineering, enhancing its durability emerges as a crucial element in the development of sustainable structures characterized by increased life-span and lowered carbon footprint. Zeolite has the potential to be a significant factor towards achieving this goal. In addition to its pozzolanic activity, which makes it a desirable substitute for cement, the structure and characteristics of zeolite make it attractive for its use in cementitious materials. Natural zeolites exhibit intriguing properties related to ion exchange and water transport, which are derived from their framework-like structure [12,39].

The durability of concrete denotes its capacity to withstand and resist the action of different factors that can affect its structural integrity and functional performance. Enhancing this property will contribute to increasing the resilience of structures in line with sustainable construction practices, thereby reducing the environmental impact.

The ability of concrete to withstand frost effects is a critical characteristic as the material possesses a specific pore structure and water content. The repeated freezing and thawing cycles, as well as the presence of de-icing salts, can lead to the deterioration of the material [40]. Specific mechanisms have been suggested, stemming from the increase in volume of around 9% that the water undergoes upon freezing. Internal stresses will appear due to the hydraulic pressure inside the pores, while repeated freezing and thawing will lead to additional water in the capillary pores as well [41]. Following this process, which starts at the surface and propagates inwards, the microstructure can be severely affected.

Zeolites have been studied in relation to their potential for enhancing the frost resistance of concrete. Substituting 10% of the cement mass with natural zeolites reduces the percentage of strength loss after 150 cycles, when compared to the initial value, by over 30% [7]. Additionally, the study indicated that incorporating an air-entraining agent further enhanced resistance in both zeolite and control mixes, with the zeolite showing a positive impact in this comparison as well.

Several replacement percentages of cement with natural zeolite (10, 20, 30 and 40%) were investigated in [42]. It was found that only specimens with 10% and 20% replaced cement have a higher frost resistance coefficient when compared to the control. The study also found the same behavior for the mass loss due to de-icing salts. Weight loss during freezing–thawing was also found to be reduced by over 80% when using 15% zeolite instead of cement [43].

Research on mortars revealed that only 5% cement replacement results in better values than the reference [44], while in another study it was found that 10% increases frost resistance [45]. Both studies agree that superior replacement values will lead to poor resistance to the freezing–thawing of mortars. The researchers explain this on the basis of the pozzolanic activity of zeolite, which will lead to a densification of the microstructure, thus prohibiting expansion of ice crystals and subsequent damage.

Chloride ions are mainly dangerous for reinforced concrete, with reinforcing bars being subjected to corrosion. Depletion of the passive layer will lead to the oxidation of steel, which leads to its volume expanding and, consequently, to the cracking of concrete due to internal stresses [46]. Several studies have indicated that zeolite can effectively decrease the diffusion of chloride ions.

It was found that replacements of 15% and 30% cement by natural zeolites significantly reduce chloride ion permeability compared to control, with values reduced by around 90% at an age of 90 days [11]. In another study where lower concentrations of zeolites were used (10% and 15%), chloride resistance was found to decrease, but with a maximum of 70% [34]. However, the authors simultaneously substituted volcanic tuff for the fine aggregates while also replacing cement.

In a more detailed study, chloride profiles and the total and surface chloride concentration, as well as the apparent chloride diffusion coefficient, were investigated [47]. These were tested in splash, tidal and laboratory conditions. Zeolites were used in percentages of 10%, 15% and 20% replacing cement. The chloride profiles were all steeper (for all types of tests and all concentrations) than those obtained for control. In tests under laboratory and tidal conditions, the total chloride contents at 10mm were similar for all zeolite concretes and the values were lower by around 20% than the reference, while, under splash conditions, only the specimens with 20% zeolites showed a reduction of over 50%. The apparent diffusion coefficient was markedly lower for all zeolite specimens, while the surface chloride concentration was lower only for the 20% replacement level.

Another study on the same replacement levels of 10%, 20% and 30% of cement with zeolites determined that the diffusion coefficient was lowered by more than 50% in modified concrete mixes [48]. It was also found that 10% and 20% zeolite use as a cement replacement leads to a decrease in the diffusion coefficient of more than 100%, as highlighted in [49]. Ahmadi replacement levels of 10%, 15% and 20% reduce the apparent diffusion coefficient by up to 66%, while 5% cement substitution shows almost no change [2].

The positive influence of zeolites regarding chloride diffusion can be altered by other factors, such as the water to binder ratio (w/b) and temperature and age of the specimens.

Tests performed on high cement replacement percentages, namely 30% and 40%, applied on mixes with w/b of 0.3 and 0.4, showed that only the lower w/b value had reduced chloride diffusivities [50]. In this case, 30% showed a greater chloride diffusivity reduction than 40% (38% vs. 21%) did compared to control. A similar approach was taken by other researchers, who tested various w/b (0.3, 0.35, 0.4 and 0.45) with cement replacement levels of 10% and 15% [17]. Irrespective of water or zeolite content, all modified concrete mixes showed reduced chloride permeability, with higher zeolite levels showing more improvement.

The influence of curing age was also investigated for a 15% cement replacement percentage with natural zeolite [43]. The migration coefficient up to 365 days and the behavior of control, which was quite different compared to the zeolite-containing concrete, were measured. The reference had a more linear drop in the migration coefficient with age, while the modified concrete showed a more abrupt change during the first 28 days. The final value for the reference was also 3 times higher at the end of the 365 day experiment. The same type of experiment was performed in [51] for 10% and 15% zeolite content. The same abrupt change in the migration coefficient was observed in both mixes, with slightly different slopes but with final values being similar at 365 days. The difference between the control and modified recipes is also similar to the previous study, with the control’s migration coefficient being around 3 times higher than that of the zeolite concretes.

The temperature influence was also measured in a research work trying to replicate, as much as possible, the real life exposure conditions of concrete [52]. Cement was replaced in amounts of 10%, 15% and 20% with natural zeolite, and the temperatures used in the study were: 22 °C, 35 °C and 50 °C. At every temperature, the apparent diffusion coefficient was reduced more with the increasing zeolite content. At the same time, the coefficient grew with increasing temperature for all specimens, but the increase was smaller for the zeolite concretes than the reference. The smallest increase was shown to happen for the 15% replacement.

Concrete is particularly susceptible to the action of acid environments, due to its alkaline nature. In general, any acid will firstly react with calcium hydroxide and soluble calcium salts will be formed. These are easily removed from the cement matrix, thus lowering its resistance [53]. The resulting calcium salts can be either very soluble, in the case of aggressive acids, or of lower solubility, when interaction with less aggressive acids occurs. One of the most aggressive and damaging instances of acid attack is represented by the interaction of concrete with sulfuric acid. This is due to the fact that the resulting salt, namely calcium sulfate (gypsum), will further react with calcium silicate hydrate (CSH), causing serious structural damage, or with calcium aluminate, which results in ettringite, that has a larger volume than gypsum and will induce micro-cracks [54,55].

Scientific literature findings vary in their assessment of the impact of zeolite utilization on the acid attack resistance of concrete, tending to predominantly indicate a detrimental effect. When testing concrete specimens with 15% and 30% cement substitution by natural zeolite [11], the results indicated that while the weight of the control mix increased during a 300-day immersion period, the mass of the zeolite concrete specimens initially decreased and then returned close to the initial value. However, the residual compressive strength was significantly affected by acid attack in the presence of zeolite. The initial strength was reduced by 20.8% (15% zeolite) and 23.3% (30% zeolite), which was considerably higher than the loss experienced by the control mix (5.5%).

Strength loss after sulfuric acid immersion was also obtained for higher zeolite contents, replacing 30% and 40% of cement [50]. Two water to binder (w/b) ratios: 0.3 and 0.4, were considered and the strength loss was more pronounced for the higher w/b for all mixes. The zeolite specimens exhibited a greater reduction in compressive strength compared to the reference, with the difference being more pronounced for w/b = 0.4. Additionally, it was found that the mass loss was lower for the 30% mixture compared to the reference for both w/b values, while the 40% mixture was more adversely affected in comparison to the control.

In the case of lower concentrations of zeolite in concrete (10% and 15%), it was found that, in relation to sulfuric acid, using zeolite increased the depth of erosion. Results indicated loss in both mass and strength proportional to the amount of zeolite [34]. The authors also performed tests with hydrochloric acid, for which zeolite imposed a similar trend of mass loss, albeit at lower reduction values.

For the same cement substitution amounts of 10% and 15%, opposing results after performing sulfuric acid immersion for 8 weeks were obtained in a different study [56]. In this case, the weight loss of 10% zeolite concrete was similar to the reference mix, while the 15% zeolite specimens presented a 40% lower weight loss. The same trend was observed in the results showing load loss by splitting. The 10% mix showed similar results to the control mix after 4 weeks immersion, but the loss was 35% smaller after 8 weeks. In contrast, the 15% zeolite mix demonstrated a load loss at 4 weeks that was 10 times smaller than that of the reference and nearly 4 times smaller at 8 weeks. These findings are consistent with the results reported in [57], where positive outcomes for specimens containing 20% cement replacement were observed. This study focused on resistance measurements that indicated a lower potential for corrosion for the zeolite mixes.

The movement of water within concrete is a critical factor in determining its long-term durability as it influences the penetration and distribution of chemical substances. The consequences of water ingress and permeation can be either advantageous or detrimental. For instance, the saturation of capillary pores due to water penetration can result in reduced resistance to frost. Additionally, the transport of water within concrete can act as a barrier against harmful gases, while also serving as a medium for the movement of various ions. Moreover, the presence of water may lead to its absorption by ettringite and alkali silica gel, causing volume expansion.

In most situations, the replacement of cement with zeolite has been demonstrated to enhance the performance of concrete in terms of water transport properties.

Water penetration is primarily influenced by the microstructure of the material, particularly the pore structure or network. Literature reports indicate that using zeolite decreases this parameter. When testing mixes with zeolite substituting 15% and 30% cement, it was found that water penetration depth decreased by up to 26% with increasing zeolite content at both 28 and 90 days when compared to the reference mix [11]. Similarly, in another study, higher decreases in water penetration values, tested at 28 days, when using 10% and 15% zeolite, with differences of 55% and 65%, respectively, were reported [51]. Using the same replacement values of 10% and 15% and changing the w/b ratio (0.35, 0.40, 0.45, 0.50), it was found that the penetration depth decreased with increasing zeolite content, while the w/b ratio had the opposite effect [17]. A smaller decrease was found when replacing 10% of the cement with zeolite, for which the water penetration depth reduced by only 13% [7]. Nevertheless, in this study, the reference mix already had a very low penetration depth. All of these results consistently indicated a relationship between reduced water penetration depth and other durability factors related to pore structure, including frost resistance and chloride diffusion/migration.

There is a more substantial body of research regarding the water absorption properties of mixtures incorporating zeolite, yielding a range of findings. However, the majority of studies tend to support the beneficial impact of zeolite use on water absorption.

An increase in water absorption of around 20% when using 15% and 30% cement replacement percentages was found. This behavior was assumed to be caused the higher absorption properties that zeolite particles have compared to cement [11]. Similarly, a nearly 23% increase in water absorption when substituting 10% of cement was reported in [7]. In a long-term study on concrete with zeolite-replaced cement in amounts of 10% and 15%, it was concluded that modified concrete presented higher water absorption than the control mix [51]. The authors tested the behavior of specimens in two circumstances: after immersion and after immersion and boiling. The results showed that at 365 days, absorption was higher by around 12% and 25% for 10% and 15% zeolite, respectively, in both scenarios. The findings correlated to permeable pores measurements that presented a similar behavior to water absorption.

Reports of water absorption decreasing with zeolite use are, nevertheless, more numerous. In this context, a decrease of up to 35% when cement was replaced with zeolite at levels of 10% and 15% was observed [34]. In this study, a part of fine aggregates was also replaced by tuff. The authors attributed this decrease to the pozzolanic activity of zeolite, which modified the capillary pore structure. The findings also had a good correlation with chloride diffusion measurements. A non-linear relation between cement replacement values with zeolite (10%, 20% and 30%) and water absorption was reported in [48]. Zeolite in amounts of 10% and 20% showed a slight increase in water absorption, with values close to the reference. Nevertheless, the replacement value of 30% proved to lower water absorption by around 15%. Water absorption measurements demonstrated a similar behavior to results obtained for the volume of voids. Lower substitution values (5%, 10%, 15%, 20%) were considered and all zeolite concrete mixes exhibited similar values of water absorption, which were around 20% lower than the reference [2]. The same replacement values but using two w/b ratios: 0.38 and 0.45, were considered in a different study [28]. Water absorption values at 90 days for high w/b = 0.45 were in good agreement with the previous study, with the variation in zeolite percentage having little impact. Nonetheless, using a lower w/b of 0.38 leads to a constant decrease in water absorption with zeolite content, reaching a reduction of almost 50% compared to control in the case of 20% zeolite use. A reduction in water absorption was also observed when using 10% and 20% zeolite instead of cement [49]. A higher zeolite percentage resulted in a slightly higher decrease in water absorption of about 20%. Lower zeolite concentrations, replacing cement with 2.5%, 5%, 7.5% and 10% zeolite, were also considered [30]. While 2.5% and 5% zeolite use yielded similar values to control, water absorption was significantly reduced when using 7.5% and 10%, by over 50%.

Sorptivity represents a durability factor that defines the absorption by capillary forces which can also be related to other durability parameters. Overall, zeolite use as cement replacement helps reduce this type of transport, albeit up to a certain concentration. This was demonstrated in tests using 10%, 20% and 30% replacement levels [48]. While sorptivity increased with increased zeolite content, 10% and 20% zeolite use provided smaller values than the reference mix. The reduction in capillary absorption observed in specimens with 15% cement replaced by natural zeolite was attributed to the pozzolanic activity of zeolite, leading to the formation of secondary CSH [43,51]. Additionally, when the w/b ratio was varied, a diminished benefit from zeolite usage was reported with the increase in the w/b ratio.

The phenomenon of carbonation is the chemical process initiated by the interaction of cement paste with carbon dioxide. This reaction has the potential to corrode reinforcement by lowering the pH of concrete to levels as low as 8.3, which is below the depassivation threshold [58]. Due to carbonation, the porosity of cement pastes is bound to change and dissolution of certain cement phases is expected. At the same time, structural changes in C-S-H may result in strength increases, followed by carbonation cracking. Carbon dioxide affects both calcium hydroxide and CSH gels, leading to a reduction in porosity [59].

Accelerated carbonation tests were conducted on cement replacement levels of 10% and 15%, revealing a significant rise in carbonation depth with the incorporation of zeolite [17]. The measurements were performed at 28, 90 and 270 days, indicating a substantial reduction in carbonation depth after 90 days for all samples. Additionally, various w/b ratios of 0.35, 0.40, 0.45 and 0.50 were employed, demonstrating that an increase in w/b ratio correlates with higher carbonation depth. The findings were assumed to be related to the consumption of CH due to the pozzolanic reaction of natural zeolite. This significantly reduced the reaction between the carbon dioxide and the available CH and resulted in a larger carbonation depth.

Similar tests were conducted on the same cement substitution rates (10%, 15%), as reported in another study [56]. Their investigation demonstrated that zeolite concrete mixes exhibited increased carbonation depths under accelerated carbonation conditions, with values directly correlated to the extent of cement replacement. Additionally, the study included an assessment of natural carbonation effects, which indicated that only the 15% zeolite substitution level resulted in a measurable carbonation depth.

Instead of serving as a parameter, electrical resistivity functions as an indicator of concrete durability, reflecting the ease of ion transportation within the material being measured. This property is linked to several durability factors, including carbonation, frost resistance, acid attack, and corrosion of reinforcement [60].

The findings from various studies indicate that the substitution of cement with zeolite results in an increase in resistivity and resistance values. It was observed that concrete mixes containing 10%, 20% and 30% zeolite in place of cement exhibited higher resistivity compared to the reference mix at both 28 and 365 days [48]. The highest resistivity value was recorded for the mix with 30% zeolite, showing a four-fold increase compared to the control mix. Similarly, a 3.5-fold increase in resistivity when 15% zeolite was used instead of cement was reported in another study [43]. Laboratory investigations on concrete incorporating lime and 10% and 15% zeolite as cement replacements showed resistivity increases of 2 and 2.75 times compared to the reference mix at 365 days [51]. It is noteworthy that measurements of permeable pores exhibited a similar trend to electrical resistivity.

The impact of zeolite replacement at 10% and 20% levels was also assessed [49]. After 90 days, the measurements indicated that the resistivity of the specimens was 3.5 and 6 times higher than the control group, respectively. Different zeolite percentages were considered, 10% and 15%, for which a significant increase in resistivity with age for these samples up to 270 days, compared to a smaller increase in the control group, was observed [17]. Additionally, their findings suggested that a higher zeolite content led to greater resistivity. Furthermore, the study revealed a general decrease in resistivity with an increase in the water-to-binder ratio for all specimens. In a study encompassing a broader range of cement replacement percentages (5%, 10%, 15%, 20%), it was found that all specimens containing zeolites exhibited higher resistivity values compared to the control, with the 5% zeolite mix closely resembling the reference [2].

It is important to note that there exists a minimum resistivity threshold that serves as a critical limit for the onset of reinforcement corrosion. Consequently, a resistivity level above 20kΩ·cm is considered adequate for safeguarding against corrosion [49]. It is noteworthy that the electrical resistivity results for the zeolite mixes mentioned earlier all surpass the minimum value required for cement replacement percentages, exceeding 5%. Some of the presented reports indicate that certain control mixes do not meet this criterion.

The presence of a moisture gradient in concrete, caused by uneven drying, induces internal stresses in the material, which can lead to cracking in certain circumstances [61]. While this issue has been extensively researched in conventional concrete for more than five decades, there is limited documentation on its occurrence in zeolite-containing concrete.

Najimi et al. conducted a study to investigate the impact of replacing 15% and 30% of cement with natural zeolite on the drying shrinkage of concrete [11]. The specimens were cured in water for 28 days, and the drying shrinkage was subsequently measured. The results revealed a reduction of 16% and 36% in drying shrinkage for the concrete samples with 15% and 30% zeolite replacement, respectively, compared to the control group at 90 days. This difference was further amplified at 120 days. The observed outcomes were found to be closely associated with the decrease in moisture content.

A more consistent decrease was observed in a study where 10% cement was substituted with zeolite [7]. The modified concrete exhibited nearly three times lower drying shrinkage compared to the control, and a similar strong association with water loss was noted.

Similar studies were conducted on high-performance and ultra-high-performance concrete. The internal curing capabilities of zeolites contributed towards a significant reduction in the autogenous shrinkage [5,26,62].